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EM 1110-2-2300
31 Jul 94

saddles may provide a spillway location. If the reservoir be constructed, it may be desirable to combine the
rim is high and unbroken, a chute or tunnel spillway may spillway and dam into one structure, indicating a concrete
be necessary (Bureau of Reclamation 1984). overflow dam. In some cases where required excavation
from the spillway channel can be utilized in the dam
c. Geology and foundation conditions. The geology embankment, an earth or rock-fill dam may be advanta-
and foundation conditions at the damsite may dictate the geous (Golze 1977, Bureau of Reclamation 1984).
type of dam suitable for that site. Competent rock
foundations with relatively high shear strength and resis- f. Environmental. Recently environmental consid-
tance to erosion and percolation offer few restrictions as erations have become very important in the design of
to the type of dam that can be built at the site. Gravel dams and can have a major influence on the type of dam
foundations, if well compacted, are suitable for earth or selected. The principal influence of environmental con-
rock-fill dams. Special precautions must be taken to cerns on selection of a specific type of dam is the need to
provide adequate seepage control and/or effective water consider protection of the environment, which can affect
cutoffs or seals. Also, the liquefaction potential of gravel the type of dam, its dimensions, and location of the spill-
foundations should be investigated (Sykora et al. 1992). way and appurtenant facilities (Golze 1977).
Silt or fine sand foundations can be used for low concrete
(or roller-compacted concrete) and earth dams but are not g. Economic. The final selection of the type of
suitable for rock-fill dams. The main problems include dam should be made only after careful analysis and com-
settlement, prevention of piping, excessive percolation parison of possible alternatives, and after thorough eco-
losses, and protection of the foundation at the downstream nomic analyses that include costs of spillway, power and
embankment toe from erosion. Nondispersive clay foun- control structures, and foundation treatment.
dations may be used for earth dams but require flat
embankment slopes because of relatively low foundation 2-6. Environmental Conditions
shear strength. Because of the requirement for flatter
slopes and the tendency for large settlements, clay foun- This policy applies to all elements of design and construc-
dations are generally not suitable for concrete (or roller- tion. Actions to be taken in some of the more important
compacted concrete) or rock-fill dams (Golze 1977, areas are:
Bureau of Reclamation 1984).
a. Overflow from slurry trench construction should
d. Materials available. The most economical type not be permitted to enter streams in substantial quantities.
of dam will often be one for which materials can be Settling ponds or offsite disposal should be provided.
found within a reasonable haul distance from the site,
including material which must be excavated for the dam b. Borrow areas must be located, operated, and
foundation, spillway, outlet works, powerhouses, and drained to minimize erosion and sediment transport into
other appurtenant structures. Materials which may be streams.
available near or on the damsite include soils for embank-
ments, rock for embankments and riprap, and concrete c. Alterations to the landscape caused by clearing
aggregate (sand, gravel, and crushed stone). Materials operations, borrow area operations, structure excavations,
from required excavations may be stockpiled for later use. and spoil areas must be controlled and treated by final
However, greater savings will result if construction sched- grading, dressing, turfing, and other remedial treatments
uling allows direct use of required excavations. If suit- as to minimize and eliminate adverse postconstruction
able soils for an earth-fill dam can be found in nearby environmental effects, as well as to eliminate unsightly
borrow pits, an earth dam may prove to be more econom- areas and promote aesthetic considerations. General state
ical. The availability of suitable rock may favor a rock- and local requirements on erosion control, dust control,
fill dam. The availability of suitable sand and gravel for burning, etc. should be followed. Such postconstruction
concrete at a reasonable cost locally or onsite is favorable alterations planned for these purposes must be compatible
to use for a concrete (or roller-compacted concrete) dam with the requirements of safety and performance of the
(Golze 1977, Bureau of Reclamation 1984). dam.

e. Spillway. The size, type, and restrictions on d. Study with a view to their elimination must be
location of the spillway are often controlling factors in the given to other potentially undesirable by-products of con-
choice of the type of dam. When a large spillway is to struction operations related to the particular environment

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